Guidebook-documented markdownlint-cli2 pre-commit hook is not set-up in boilerplate

The guidebook currently documents adding the markdownlint-cli2 pre-commit hook, however this is currently not added in the boilerplate.

Also, the markdownlint-cli2 defaults are rather strict, so we should add a .markdownlint-cli2.yaml with some agreed sensible configuration options (particularly, line_length: 80 seems a bit harsh.